152 ANIMAL KINGDOM. 



of a series of vertebrae, and is hence composite in the skeleton 

 but essentially simple in the nervous system. 



V. Protozoa. — Besides the species above included, there are others, 

 of extreme simplicity of structure, which are sometimes referred 

 to the Eadiates and sometimes to a separate group, called Protozoa 

 (from 7rpuTog, first, and £uov, animal). They embrace some of the 

 microscopic organisms or animalcules, and also the Sponges. They 

 have, in general, no proper mouth or stomach. 



Subdivisions or the Sub-Kingdoms. 

 [Details relating to the higher groups are given as follow : Fishes, 

 p. 277 ; Reptiles, p. 343 ; Mammals, p. 421.] 



I. Vertebrates. 



Four classes are generally recognized : — 



1. Mammals. — Species suckling their young, — a characteristic 

 peculiar to this highest branch of the animal kingdom : all are 

 warm-blooded and air-breathing. Examples: ordinary Quadrupeds, 

 large and small, with Whales and Seals. 



2. Birds. — Warm-blooded and air-breathing ; oviparous ; covered 

 with feathers, and adapted for flying. 



3. Reptiles. — Cold-blooded, air-breathing; oviparous; skin naked 

 or covered with scales, as the Crocodile, Lizard, Frog. 



4. Fishes. — Cold-blooded ; breathing by means of gills ; skin naked 

 or covered with scales. 



II. Articulates. 



The classes are five in number, — three of them — Insects, Spiders, 

 and Myriapods — aerial in respiration ; the other two — Crustaceans and 

 Worms — breathing by means of gills, and living in water or moist 

 earth. 



A. Eespiration through breathing-holes (spiracles) along the sides 

 or posterior part of the body ; admitting air to circulate in the 

 interior. Essentially land or aerial species. 



1. Insects. — The body in three parts, — a head, thorax, and abdomen 

 distinct ; only three pairs of legs. Examples: the Beetle, Wasp, Fly, 

 Butterfly. 



2. Spiders. — The body in two parts, the head and thorax not dis- 

 tinct; four pairs of legs. Examples: the Spider, Tick, Scorpion. 



3. Myriapods. — The body worm-like in form, the head not promi- 

 nently distinct from the rest; legs numerous. Examples: the Cen- 

 tipede. 



